Mysql if statement if one colum isn't empty, compare two others
SELECT c.* FROM cca_coupons c
JOIN cca_owners o ON o.id = c.owner_id
WHERE o.id='1'
  AND IF c.max_redemptions != '0' THEN c.amount_generated >= c.max_redemptions

SELECT c.*
FROM cca_coupons c
JOIN cca_owners o ON o.id=c.owner_id
WHERE c.amount_generated >= c.max_redemptions
AND c.max_redemptions <> '0'